1. Field of the Invention
The various aspects of the invention relate generally to discharge lamps, and more specifically to inductively coupled electrodeless lamps. The invention also relates to novel lamp configurations, coupling circuits, bulbs, heat dissipating lamp head assemblies, RF sources (oscillators), directional couplers, aperture structures, and excitation coils for inductively coupled electrodeless lamps. Another aspect of the invention also relates to an improved electrodeless aperture lamp, and to methods of making and using an electrodeless aperture lamp. The invention also relates generally to a novel high power, high frequency solid state oscillator. The invention further relates to a novel control circuit and method for operating an electrodeless lamp. Another aspect of the invention relates generally to RF driven loads with changing impedance characteristics.
2. Related Art
In general, the various aspects of the invention relate to the type of lamps disclosed in U.S. Pat. Nos. 5,404,076 and 5,903,091 and PCT Publication No. WO 99/36940, each of which is herein incorporated by reference in its entirety.
Electrodeless discharges are generally classified as either E discharges, microwave discharges, travelling wave discharges, or H discharges. Most examples of the invention relates to those discharges preponderantly characterized as H discharges.
FIG. 1 is a schematic diagram of a conventional electrodeless lamp which produces an E discharge. A power source 1 provides power to a capacitor 2. A gas-filled vessel 3 is placed between the plates of the capacitor 2. E discharges in electrodeless lamps are similar to arc discharges in an electroded lamp, except that current is usually much less in an electroded arc discharge. Once breakdown of the gas to its ionized or plasma state is achieved, current flows through the capacitance of the vessels walls between the plates of the capacitor 2, thereby producing a discharge current in the plasma.
FIG. 2 is a schematic diagram of a conventional electrodeless lamp which produces a microwave discharge. A microwave power source 4 provides microwave energy which is directed by a waveguide 5 to a microwave cavity 6 which houses a gas-filled bulb 7. The microwave energy excites the fill in the bulb 7 and produces a plasma discharge. In a microwave discharge, the wavelength of the electromagnetic field is comparable to the dimensions of the exciting structure, and the discharge is excited by both E and H components of the field.
FIG. 3 is a schematic diagram of a conventional electrodeless lamp which produces a travelling wave discharge. A power source 8 provides power to a launcher 9. A gas-filled vessel 10 is disposed in the launcher 9. The gap between the electrodes of the launcher 9 provides an E field which launches a surface wave discharge. The plasma in the vessel 10 is the structure along which the wave is then propagated.
FIG. 4 is a schematic diagram of a conventional electrodeless lamp which produces an H discharge. Electrodeless lamps which produce an H discharge are also referred to as inductively coupled lamps. As shown in FIG. 4, one example for a conventional inductively coupled lamp includes a low frequency power source 11 providing power to a coil 12 which is wound around a gas-filled vessel 13. The alternating current in the coil 12 causes a changing magnetic field, which induces an electric field which drives a current in the plasma. In effect, the plasma can be analyzed as a single turn secondary to the coil 12. An H discharge is characterized by a closed electrical field, which in many examples forms a visible donut-shaped plasma discharge.
A number of parameters characterize highly useful sources of light. These include spectrum, efficiency, brightness, economy, durability (working life), and others. For example, a highly efficient, low wattage light source with a long working life, particularly a light source with high brightness, represents a highly desirable combination of operating features. Electrodeless lamps have the potential to provide a much longer working life than electroded lamps.
One aspect of the invention is to provide an ultra bright, low wattage electrodeless lamp which has many practical applications. Specifically, an aspect of the invention is to provide an electrodeless aperture lamp which is powered by a solid state RF source in the range of several tens to several hundreds of watts. Various aspects of the invention may be adapted to provide an excellent light source for such diverse applications as projection display, automotive headlamps and general illumination including office environments, schools, factories, shops, homes, and virtually anywhere which requires or benefits from artificial lighting.
According to one aspect of the invention, an inductively coupled electrodeless lamp includes an excitation coil; a capacitor structure connected to the excitation coil, the capacitor structure and excitation coil together forming a resonant lamp circuit; an electrodeless lamp bulb positioned proximate to the excitation coil, the bulb containing a fill which emits light when excited by RF energy; and an RF source connected to the resonant lamp circuit and adapted to provide RF energy for exciting the fill, wherein the capacitor structure is adapted to inhibit arcing during operation of the lamp. For example, the excitation coil comprises a wedding ring shaped excitation coil having an axial lead on one end and a radial lead on the other end, and wherein the capacitor structure comprises a capacitor stack connected to the axial lead of the wedding ring coil. The capacitor stack may include a material having a low dielectric constant for the high voltage capacitor an may further include a conformal coating covering at least a portion or substantially all of the capacitor stack and optionally a portion of the axial lead of the wedding ring coil. The capacitor stack may have a circular high voltage plate, which may include an edge radius which is larger than one half of the plate thickness. In some examples, the lamp may include a heat transfer structure providing a thermal conduction path from the capacitor structure to a heat dissipating structure.
According to another aspect of the invention, the capacitor structure comprises a coaxial capacitor circuit, including a first capacitor comprising a first cylindrical sleeve; a second capacitor comprising a second cylindrical sleeve disposed at least partially inside the first cylindrical sleeve of the first capacitor; and insulators disposed in between the first and second sleeves, wherein the first and second capacitors are connected in series with a center conductor being connected at a junction of the series connection.
According to another aspect of the invention, the lamp includes an enclosure housing the resonant lamp circuit, the enclosure comprising thermally conductive structures for transferring heat from the lamp circuit, and the enclosure comprises substantially flat outer surfaces for interfacing with further heat dissipating structures. The excitation coil may be made from copper. In some examples, the enclosure comprises a base portion and a cover, and a thermal gasket is disposed between the cover and the base. The coil and capacitor structure may be integrated in a single assembly, with the capacitor structure including a multi-layer printed circuit board adapted to form a capacitor stack.
According to yet another aspect of the invention, an inductively coupled electrodeless lamp includes an excitation coil; a capacitor structure connected to the excitation coil, the capacitor structure and excitation coil together forming a resonant lamp circuit; an electrodeless lamp bulb positioned proximate to the excitation coil, the bulb containing a fill which emits light when excited by RF energy; an RF source connected to the resonant lamp circuit and adapted to provide RF energy for exciting the fill; and a structure encasing the bulb except for a light emitting aperture, the structure comprising a ceramic material configured to promote heat transfer away from the bulb along a thermal path other than radially with respect to an axis of the coil. For example, the ceramic material comprises a high thermal conductivity material. In some examples, the material exhibits relatively higher thermal conductivity along a direction and the material is adapted such that the direction of higher thermal conductivity is aligned with an axis of the coil. For example, the material comprises boron nitride. The lamp may further include an enclosure housing the resonant lamp circuit, and the structure may include a ceramic cup with a flange, where a resilient, thermally conductive material is disposed between the flange and a heat dissipating structure inside the enclosure.
According to other aspects of the invention, the aperture structure includes a ceramic cylindrical rod defining a cavity at one end which is adapted to receive the bulb, wherein the bulb is disposed in the cavity; and a ceramic washer defining an aperture and disposed against the bulb, whereby the bulb is cooled relatively more from the portion of the bulb opposite from the aperture. In some examples, the structure includes a relatively tall cylindrical and hollow structure adapted to support a bulb along its axial dimension so that at least a portion of the cylindrical cup extends significantly beyond the bulb in each axial direction. In other examples the bulb bears a high temperature, high reflectivity, and wide angle dichroic coating except in a region which defines the aperture, and the structure comprises a high thermal conductivity ceramic encasing the bulb except for an opening in the region of the aperture.
According to yet another aspect of the invention, an oscillator includes an amplifier having an input and an output; and an impedance transformation network connected between the input of the amplifier and the output of the amplifier, the impedance transformation network being configured to provide suitable positive feedback from the output of the amplifier to the input of the amplifier to initiate and sustain an oscillating condition, the impedance matching network being further configured to protect the input of the amplifier from a destructive feedback signal, wherein the impedance transformation network comprises dual asymmetrical feedback paths adapted to provide an increased tuning range as compared to dual symmetrical feedback paths. For example, the amplifier comprises two RF power FET transistors connected in parallel and configured with soft gate switching. In some examples, the oscillator further includes a gate pad with a perpendicular transmission line extending therefrom and forming a resonant xe2x80x9cTxe2x80x9d, and the feedback network is attached to the leg of the resonant xe2x80x9cTxe2x80x9d. The oscillator may further include a continuously variable tuning circuit for adjusting the operating frequency of the oscillator. For example, the tuning circuit consists of solid state electrical components with no mechanically adjustable devices. In some examples, the tuning circuit comprises a plurality of PIN diode circuits configured as voltage controlled resistors. In other examples, the tuning circuit comprises a complementary PIN diode circuit. The oscillator may further include a heat transfer structure providing a thermal conduction path from the PIN diode to a heat dissipating structure. For example, the heat transfer structure comprises a metal post soldered to one pad of the PIN diode and the heat dissipating structure comprises an electrically grounded heat spreader plate.
In some oscillator examples, the impedance transformation network is adapted to combine a first portion of feedback from a load connected to the oscillator with a second portion of feedback from the amplifier to control a relative angle between lines of constant current and lines of constant frequency as plotted on a Rieke diagram.
In other oscillator examples, the oscillator includes a load connected to the oscillator; at least one impedance element connected to either the oscillator or the load by a switch; and a control circuit adapted to operate the switch at least once during operation of the oscillator. For example, the control circuit is adapted to operate the switch a pre-determined amount of time after the oscillator is started. Alternatively, where the load comprises an electrodeless discharge lamp, the control circuit may be adapted to operate the switch based on a sensed lamp condition. In some examples, the control circuit is adapted to operate the switch in accordance with providing closer matching of an impedance of the oscillator and the load during starting. In other examples, the control circuit is adapted to operate the switch in accordance with avoiding a region of unstable oscillator operation during starting.
According to a still further aspect of the invention, a lamp apparatus includes a discharge lamp; an RF power source connected to the discharge lamp for providing RF power to the lamp; and an RF control circuit adapted to control an operating parameter of the RF power source during operation. In some examples, the operating parameter corresponds to a frequency of the RF power source, and the lamp further includes a six port directional coupler connected in between the RF power source and the discharge lamp, the six port directional coupler being configured to detect forward and reflected power and provided respective signals representative thereof, and the RF control circuit is configured to receive the signals representative of forward and reflected power and to adjust an operating frequency of the RF power source in accordance with the received signals. The control circuit may be configured to delay initiation of active control until after the oscillator starts. The control circuit may be configured to step an operating frequency of the oscillator through a range of frequencies until the lamp is determined to be operating at a resonant frequency. The control circuit may be configured to adjust an operating frequency of the oscillator to minimize reflected power.
In other examples, the operating parameter corresponds to an amount of RF power coupled to the discharge lamp during operation. For example, the RF control circuit may be adapted to provide less RF power to the lamp prior to ignition as compared to an amount of RF power provided during steady state operation, thereby reducing arcing potential and reflected power during starting. The RF control circuit may also be adapted to temporarily provide more RF power to the lamp following ignition as compared to an amount of RF power provided during steady state operation, thereby bringing the lamp to full output faster. The RF control circuit may also be adapted to adjust a supply voltage of the RF power source during steady state operation to provide at least one of substantially constant forward power and substantially constant light output.
In other examples, the operating parameter corresponds to an amount of gate bias current provided to an active element of the RF power source. For example, the gate bias current is controlled such that the RF power source is not turned on until other functions of the RF control circuit have initialized.
One example of a lamp head according to an aspect of the invention includes a housing having a base and a cover adapted to fit over the base, an inductively coupled electrodeless lamp circuit including an excitation coil and a capacitor assembly which form a resonant circuit, the lamp circuit further including an aperture bulb positioned proximate to the excitation coil, wherein the base is adapted to receive the electrodeless lamp circuit and the cover provides an opening for light exiting from the aperture, and wherein the housing provides radio frequency shielding and substantially flat interfaces for external heatsinking.
According to another aspect of the invention, an integrated inductively coupled electrodeless lamp circuit includes an excitation coil, a high voltage capacitor, and a low voltage capacitor in an integrated assembly, wherein the high voltage capacitor and the low voltage capacitor are formed as a stack comprising a first conductive material, a first dielectric material, a second conductive material, a second dielectric material, and a third conductive material, wherein the first conductive material comprises a lead of the excitation coil, the second conductive material comprises a common plate of the high and low voltage capacitors, and the third conductive material comprises a grounded conductive surface, and wherein the first dielectric material is positioned between the first and second conductive materials thereby forming the high voltage capacitor and the second dielectric material is positioned between the second and third conductive materials thereby forming the low voltage capacitor, and wherein the first dielectric material, the second conductive material, and the second dielectric material are integrally formed as a multi-layer printed circuit board.
The foregoing and other objects, aspects, advantages, and/or features of the invention described herein are achieved individually and in combination. The invention should not be construed as requiring two or more of such features unless expressly recited in a particular claim.